The present invention relates to brush-commutated electrical machines, in particular measures for ascertaining a rotor position or rotation speed of the electrical machine.
The use of brush-commutated electric motors as actuating motors for DC voltage applications is already known. For example, brush-commutated electric motors are used as drive motors for seat adjusters, window winders, sliding roof systems and the like in a motor vehicle. A common configuration is four- and six-pole DC actuating motors with permanent-magnet excitation comprising a commutator having two brushes.
During operation of electric motors of this kind, current ripple is produced on the supply line owing to the commutation processes. The current ripple corresponds to a periodic signal which is dependent on a rotation speed of the electric motor and of which the fundamental frequency depends on the number of commutator laminations. The current ripple can be used in sensor-free systems to detect the motor rotation speed and the rotor position.
Depending on the number of poles of the electric motor and the number of commutator laminations of the commutator, a particularly high current ripple results in the event of simultaneous commutation of the brushes, the main order (fundamental frequency) of said current ripple being given by the number of commutator laminations and the actual rotation speed of the electric motor. As an alternative, the brushes can also be arranged such that they commutate in an offset manner, as a result of which a more weakly pronounced current ripple is produced, the main order of said current ripple corresponding to a multiple of the number of the commutator laminations, generally to twice the main order, given an offset of the brushes by n+½ commutator laminations. In this case there is virtually no detectable current ripple in the order which is prespecified by the number of commutator laminations.
In order to keep electromagnetic force excitations and the associated noise emissions low, it is advantageous to select the number of commutator laminations and the number of rotor poles such that the brushes commutate alternately. On account of the low current ripple, in particular in the order of the number of commutator laminations, said brushes are not suitable for operation on controllers which use the current ripple to detect the rotation speed of the electric motor and/or the rotor position.